Automatic Train Control in Rail Rapid Transit (Part 5 Of

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Train control is the process by which the move- of the way in which functional elements interact. ment of rail rapid transit vehicles is regulated for These functional relationships are also illustrated the purposes of safety and efficiency. The process is by the diagram in figure 1. Since the purpose is only carried out by a combination of elements-some to provide the reader with a general background for men, some machines—located on the train, along understanding the nature of train control, the the track, in stations, and at remote central definitions presented here are brief and nontechni- facilities. These elements interact to form a com- cal.8 mand and control system with four major functions: Train Protection . Train Protection prevention of col- Train protection is a family of functions whose purpose is to assure the safety of train movement by lisions and derail- 9 ments, preventing collisions and derailments. rain protection functions and requirements override all . Train Operation control of train other control system functions either through movement and equipment design or, in a completely manual mode, stopping at sta- by rules and procedures. The functions that make tions, up train protection are: ● Train Supervision direction of train Train detection—monitoring of the track to movement in rela- determine the presence and location of trains; tion to schedule, Train separation-assuring that trains on the . Communication interchange of in- same track maintain a safe following distance formation among to prevent collisions; the elements of the system. Route interlocking—preventing trains on crossing, merging, or branching routes from making The train control system is analogous to the sen- conflicting (unsafe) moves that would cause a sory organs and central nervous system of the collision or derailment; human body. It senses and processes information, makes decisions, and transmits commands. Also as Overspeed protection—assuring that train speed remains at or below the commanded or in the human body, the execution of commands is 10 not a function of the train control system but of posted civil speed limit as to prevent collisions resulting from going too fast to stop other parts specialized for that purpose. For exam- within the available distance and to prevent ple, the train control system may sense train speed, = determine that it should be increased, provide an derailments due to excessive speed on curves appropriate command signal to the motors, and or through switches; monitor to see that the desired result is achieved. Train and track surveillance-observing condi- The means by which a speed change is effected, tions on and in the vicinity of the track ahead however, are not part of the train control system. of the train and monitoring safety-related con- All the equipment for getting electric power to the ditions on board the train. wayside, bringing it into the train, converting it to mechanical energy, and providing tractive effort is Train Operation external to the train control system. Similarly, the equipment to select a route for a particular train and Train operation consists of those functions transmit commands to aline switches accordingly necessary to move the train and to stop it at stations are within the train control system, but the parts of the trackwork that actually move (the switch F. For more detailed technical descriptions of train control points) are not elements of the train control system. system functions and technology, see appendices A and B. gThere is no unive~]ly accepted terminology and scheme of TRAIN CONTROL definitions for train control system functions within the transit SYSTEM FUNCTIONS industry. The terms and classification employed here are based on several sources and represent the best of current usage. Presented below is a description of the specific IOA g]os5ary of train control and rail rapid transit terms is functions performed by a train control system and provided in appendix D.

17 Operation

PROTECTION OPERATION

J I

PERFORMANCE MODIFICATION I k (Train I 1 Presence)

1 r I I 1

I L I Movement Order A Movement Report

“ To simplify the diagram, the functions of Alarmlng and Recordkeepinq are not shown to board and discharge passengers. Train move- tactically, In addition, train supervision includes ment, as controlled by train operation functions, is certain information processing and recording ac- under the direction of train supervisory functions tivities not directly concerned with train safety and and always within the constraints of train protec- movement but necessary to the general scheme of tion functions. Train operation involves the follow- operations. Train supervision functions are: ing: Speed regulation-controlling train speed, with- Schedule design and implementation—prepar- in the constraints of overspeed protection, to ing a plan of service in light of expected make the run according to schedule;ll demand, available equipment, and environmental conditions and issuing a schedule to Station stopping—bringing the train to a stop implement the plan; within some specified area in a station; Door control--opening of doors in stations to Route assignment and control--selecting and permit passengers to enter or leave the train assigning routes to be followed by trains (and and closing of doors when the train is ready to rerouting as necessary); start; l2 Train dispatching-controlling train departures Train starting—initiating train departure from a from terminals or waypoints in accordance station after the doors are closed (and pro- with the schedule; vided the train protection system permits it),13 Performance monitoring—following the Train Supervision progress of trains against the schedule by ob- Train supervision involves monitoring the move- taining periodic updates of train identity, loca- ment of individual trains in relation to schedule and tion, and destination; route assignments and overseeing the general dis- position of vehicles and flow of traffic for the Performance modification—adjusting move- system as a whole. The train supervision system ment commands and revising the schedule in may thus be thought of as making strategic deci- response to train, traffic, and environmental sions which the train operation system carries out conditions.

Alarms and malfunction recording-alerting to 11Speed regulation involves more than matching actual to malfunctions, breakdowns, or problems, and command speed. It also includes control of acceleration, jerk limiting (controlling the rate of change of acceleration), slip- recording their time, location, and nature; slide control (correction of wheel spinning during acceleration and skidding during braking), and flare-out (gradual relaxation Recordkeeping —maintaining operational logs of braking effort as the train comes to a stop). Flare-out is con- and records for business and payroll purposes, sidered by some transit engineers to be a subsidiary function of for scheduling maintenance, for ordering sup- speed regulation, and hence part of the train control system. Ac- celeration control, jerk limiting, and slip-slide control are plies and equipment, and for computing tech- regarded by transit engineers to be propulsion and braking nical statistics. system functions, but they are mentioned here because of their relationship to the train control functions of speed regulation and station stopping.

12 The mechanisms that actually open and close doors are not Communication part of the train control system, but the signals to actuate these mechanisms and the interlocks to assure that doors are closed The communication system is the means by before starting and that they remain closed while the train is in which the information needed to carry out all other motion are generated within the train control system. Because of the safety implications of door control, some transit engineers train control functions is transmitted and ex- l4 consider it to be a part of train protection. changed. This information may take any of 13Train starting is sometimes classified as part of the door several forms-voice, visual, auditory, and digital control function. It is separated here for two reasons: (1) in some automated systems, door control is automatic while train starting is retained as a manual function; (2) in manual systems, the 14On the function diagram in figure 1, communication func- door control and train starting functions are often assigned to tions are indicated by the lines connecting the boxes which different persons. represent train protection, operation, and supervision functions,

19 or analog electrical signals.l5Unlike other train con- AUTOMATION trol functions, which involve information processing and decisionmaking, communication is largely a At one time or another, all of the train control facilitative process-serving to convey information functions listed above have been performed by but without producing any unique functional out- human operators, and many still are, even in the comes of and by itself. For this reason, the most technologically advanced transit systems. categorization given below indicates not functions Theoretically, any of these functions could also be as such but major classes of information that must performed by automatic devices, and more and flow throughout the system in order for other train more have, in fact, been assigned to machines over control functions to take place: the years, Before examining the technology by which train control automation has been achieved, Train protection—information necessary to it is first necessary to consider what is meant by locate individual trains, to assure their safe automation and to clarify the terminology used in separation, to prevent overspeed, and to con- this report. trol movement at route interlockings;l6 Figure 2 is a generalized diagram of the process Command and status—information on the by which any train control function is ac- operational state of the system, command sig- complished. It involves receiving information about nals to control train and switch movement, some operational state of the system and some and feedback to determine the response of desired state. This information must then be in- system elements to command inputs;l6 terpreted—for example, by comparing the two states and deriving a quantitative expression of the Emergency—information on the nature and difference, Next, an appropriate control response to location of emergency events and summons null the difference must be selected, and some for help to elements within the transit system specific command message to the controlled ele- or to outside agencies (e.g., fire, police, medi- ment must be formulated and transmitted. A final, cal, and rescue); and all-important, step is monitoring the results of Passenger service—information relating to train the control action to ascertain that the desired service and system operation for the purpose system state or condition has been achieved. This of assisting passengers using transit facilities; last step, called feedback, provides an input signal to start the process all over again, thereby creating a Maintenance—information needed to plan or loop that permits the control process to be con- conduct preventive and corrective mainte- tinuous and adaptive.l7 nance; Business operations--operational information If all of the steps in the general sequence shown used to maintain a record of (and to plan for) in Figure 2 are performed by a human operator, the work force allocation, vehicle utilization, pro- process is called manual, even though manual ac- curement of supplies and equipment, operat- tion in the strict sense may not be involved. Thus, ing expenses, and system patronage. manual denotes a process that may include visual, auditory, and other forms of sensory perception as well as purely cognitive activities such as in- terpretation, weighing alternatives, and decision- 15Some transit engineers limit the definition of communica- making. The command output might be action to verbal or visual communication (radio, telephone, TV, complished by some manual activity such as press- and the like). Machine-to-machine communications, since they ing a button or moving a control lever, or it might tend to be very specialized, are considered part of the function take the form of a voice command or simply a nod which they serve. This seems to be unnecessarily restrictive and of the head. The essential feature of a manual proc- makes an artificial distinction between information exchange by human operators and other forms of information exchange in- ess, as the term is used here, is that all the basic con- volved in operating the system (i.e., man to machine or machine trol steps to accomplish a function are human ac- to machine). The definition offered here is generic and embraces tivities. all types of information flow, regardless of how effected. 16Customarily, this part of the communication system is completely separate from the network used for other types of infor- l~his cIescriptiOn overlooks the difference between closed- mation and is considered to be an integral part of the train pro- and open-loop control systems. For a discussion of the applica- tection system. tion of each in train control technology, see appendix B.

20 It is also possible for all of the steps in the control functions. It does not necessarily suggest a com- loop to be accomplished by some mechanical or pletely automated system. It can be applied to a electrical device. If so, the process is called auto- system where certain functions or groups of func- mated. The device need not necessarily be compli- tions are performed automatically while others are cated, nor is a computer required in order for the ap- performed manually. ATP (automatic train protec- paratus to process information and make a “deci- tion), ATO (automatic train operation), and ATS sion.” A simple junction box with a two-state logic (automatic train supervision) are used to designate circuit (ON or OFF) would satisfy the definition of major groups of functions that may be automated. an automated control device, provided no human For example, if a system is said to have ATP, it actions were required to receive and interpret input means that train protection is accomplished (either signals, select and order a response, and monitor the completely or mostly) by automatic devices without result. direct human involvement. If a system is described Between the extremes of purely manual control as having ATC consisting of ATP and some ATS, and fully automatic control, there are numerous this indicates that train protection is assured by combinations of mixed man-machine control loops. automatic devices and that train supervision is a These are called semi-automated or partially auto- mixture of manual and automatic elements. By im- mated—the terms are used synonymously to denote plication, train operation in such a system would be a process (or a system) in which there are both manual. manual and automatic elements. Thus, automation is not to be taken in an absolute, all-or-nothing While automation involves the substitution of sense. The machine can be introduced by degrees machine for human control, this does not mean that into a system to perform specific functions or parts the human operator is removed from the system of functions. When comparing parts of a train con- altogether. An automated system is not always an trol system or when comparing one system with unmanned system, even though all functions are another, it is therefore possible to speak of automa- routinely performed by machines. For instance, tion in comparative terms and to say that one is train protection and train operation may be com- more or less automated than another, depending on pletely automatic in a given transit system, but how many specific functions are performed by there could still be an operator or attendant on machines. board the train to oversee equipment operation and, most importantly, to intervene in the event of For brevity, acronyms are used to describe cer- failure or malfunction. This emergency and backup tain areas where automation is applied in train con- role is, in fact, a major type of human involvement trol. ATC (automatic train control) refers generally in even the most automated train control systems, to the use of machines to accomplish train control In all rail rapid transit train control systems now in

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operation or under development, automation is AUTOMATIC TRAIN utilized only for normal modes of operation, with manual backup as the alternative for unusual condi- CONTROL TECHNOLOGY tions, breakdowns, and emergencies. The automatic equipment that accomplishes In passing, it should also be noted that automa- train control functions is often of complex design, tion is not synonymous with remote control, even but the basic technology is quite simple. The pur- though the two may at times go hand in hand. In pose of this section is to provide an acquaintance train supervision, for example, many functions are with the fundamental elements of an ATC accomplished manually by controllers who are system—track circuits, signaling apparatus, train physically far removed from the train and wayside. operating devices, interlocking controls, and super- In central control facilities, the operators may never visory equipment, The details of this technology actually see the vehicles or track and yet perform all and the design features of ATC equipment now in or most of the functions necessary to set up routes, use in rail rapid transit systems are omitted here but dispatch trains, and monitor traffic. Conversely, are provided in appendices B and C. automated functions are often performed locally, i.e., by devices on board the train or at a station or Track Circuits switch. In general, the location of the controlling element in relation to the controlled element is in- For safety and efficient operation of a transit dependent of how the functions are accomplished. system, it is imperative to know the locations of However, it is also true that automation does facili- trains at all times. The sensing device providing this tate the process of remote control, and systems with information is the track circuit, which was invented a high level of ATC tend also to employ more over 100 years ago and has remained essentially centralized forms of train control, especially for unchanged in principle even though extensively supervisory functions. refined and modified in its engineering details.

FIGURE 3.—Simple D.C. Track Circuit

22 The track circuit is an electrical circuit consisting to break, the relay would receive no current; and of a power source, the running rails, and a signal the armature would drop just as if a train were pres- receiver (relay).l8 The track is divided into ent. A broken electrical connection, a failure of the electrically isolated segments (called blocks) by in- power source, or a burned-out relay coil would also sulated joints placed at intervals in the running have the same effect. rails. l9 This forms a circuit with a power source connected to the rails at one end of the block and a relay at the other. The relay, in turn, forms part of a second electrical circuit which has its own indepen- Wayside Signals dent power supply (commonly a battery) and in- One of the earliest types of signal devices cludes a signaling device such as wayside colored employed to control train movement, and one still lights, widely used, is the automatic wayside block signal, When no train occupies the block, the relay is It consists of a color-light signal, in appearance energized by the track circuit battery, causing the much like the traffic signal on city streets, located relay to “pick up, ” i.e., a movable element (ar- beside the track at the entrance to each block, This mature) is moved to and held electromagnetically in signal is controlled by the track circuit relay, as a position opposed to the force of gravity. This described above. The signal directs train movement closes an electrical contact in the secondary signal by displaying red, yellow, or green lights (aspects) circuit. When a train enters the block, the wheels to indicate track circuit occupancy ahead, and axles conduct electricity between the running Since it would be impractical for the train to rails, thereby short circuiting (shunting) the track creep ahead block by block, waiting to be sure each circuit and reducing the current to the relay. This block is clear before entering, the wayside signals weakens the electromagnetic force holding up the are arranged to give the operator advanced indica- armature, allowing it to drop under the force of tion of speed and stopping commands. Figure 4 is an gravity. This action opens the contact that was pre- illustration of a three-block, three-aspect wayside viously closed and closes a different contact in the signal system, This signaling arrangement tells the signal circuit. The relay, therefore, acts as a switch train operator the occupancy of the track three in the secondary signal circuit and creates one blocks ahead of the train and conveys three electrical path when it picks up and another when it different movement commands (indications)— drops. green (proceed), yellow (proceed prepared to stop at Thus, the basic principle of the track circuit is the the next signal), red (stop). shunting phenomenon produced by the train In the illustration, Train A is stopped in Block 4 wheels passing along the electrically energized run- and Train B is approaching from the rear. Since ning rails. The presence of the train is detected in there is a separation of at least three blocks between the track circuit as a reduction of electrical current, them, Train B receives a green aspect at the which-by means of the relay—is used to control entrance to Block 1, allowing it to proceed at the the secondary signal circuit and operate various maximum allowable speed. At the entrance to types of track occupancy indicators. Block Z, however, Train B receives a yellow aspect, The track circuit is designed according to the fail- indicating that the train operator should be pre- safe principle. In order for a clear (unoccupied pared to stop at the next signal because there may block) indication to be given, the track circuit must be a train ahead. At the entrance to Block 3, Train B be in proper working order. If one of the rails were is commanded to stop by a red signal aspect. When Train A leaves Block 4 and moves on to Blocks 5 and 6, the signal at the entrance to Block 3 changes 18 Track circuits may utilize one or both running rails, may to yellow and then green, allowing Train B to pro- operate on direct or alternating current, and may have ceed. electromechanical relays or solid-state electronic receivers. The type described here is a double-rail dc track circuit with a relay. The wayside signaling system is made fail-safe The other types are similar in principle and operation. 19 through design and by operating rules. Dual, or Block length in rail rapid transit systems varies considera- bly as a function of track and traffic conditions and signal sometimes triple, lamps are used to illuminate each system design. Some are as short as 40 feet; others are over half a signal aspect. Redundant power sources are some- mile long, times provided. The ultimate safeguard, however, is

23 FIGURE 4.—Three-Block, Three-Aspect Wayside Signal System

procedural. A complete failure of the signal lamps brake.20 A train entering a block in violation of the or a loss of power would result in a dark (unlighted) wayside signal indication would thus be brought to signal, which standard operating rules require the a complete stop before colliding with the train in train operator to observe as if it were a red signal. the next block regardless of what action the train operator took, or failed to take. Trip Stops In addition to protecting against rear-end colli- In the wayside signal system described above, sions, trip stops can also be used in conjunction safe train movement depends solely on the com- with the track circuits and other signal appliances to pliance of the operator with signal indications. To provide automatic protection against overspeed. For guard against error, inattention, or incapacitation of this application, a timing device is added to the cir- the train operator, wayside signals can be supple- cuit controlling the trip stop. When a train enters a mented with an automatic stop-enforcing mechanism, called a trip stop. 20An alternative system employing inductive train stops is used on main-line railroads in the United States and on rail The trip stop is a device located beside the track rapid transit systems abroad. The device is somewhat more com- at each wayside signal. The type commonly used in plex than the mechanical trip stop, but it avoids mechanical con- the United States consists of a mechanical arm that tact between a stationary wayside element and a moving train is raised or lowered in response to the track occu- and is less vulnerable to blockage by snow or debris. Both trip stops and inductive train stops have the inherent disadvantage pancy detected by the track circuit. When the arm is of requiring strict alinement of wayside devices. Further, if in the raised position, it engages a triggering device either type of device is removed, the system will operate in a on the train and actuates (trips) the emergency mode that is not fail-safe.

24 block, the trip stop at the entrance to the next block of view, shows indicator lights similar to those of is in the raised position but will be lowered after a wayside signals, e.g., red, yellow, and green aspects. time interval corresponding to the minimum time Cab signals can thus convey the same movement (the maximum speed) permitted for a train to tra- commands as wayside signals, but they do so con- verse the block. This arrangement is commonly tinuously in response to the instantaneous condi- used on curves, downgrades, and other such section of the track ahead. They can also convey pre- tions of track where excessive speed could cause a cise speed commands instead of just stop-and-go in- derailment. A variation of this scheme is commonly formation, thus providing more flexible operation used at stations to allow a following train to close in and paving the way to ATO. The cab signal unit has on a leading train, provided the follower moves at an audible warning that sounds whenever the sig- appropriately diminishing speed as it approaches its nal aspect becomes more restrictive and continues leader. to sound until the operator silences it by an acknowledging device. Figure 5 is an illustration of Like track circuits and signals, the trip stop is a typical cab signal. designed to operate in a fail-safe manner. The trip is raised to the stopping position by gravity or a heavy Transferring the display of information from the spring and lowered by a pneumatic or electric wayside to the cab involves an alternate type of mechanism. Thus, failure of the trip stop actuating track circuit technology. To operate cab signals, the mechanism or its source of energy will result in the current passing through the track circuit (usually trip stop being raised to the stop position. a.c. is not steady, as for conventional wayside signals, but is pulsed (turned on and off) at several Cab Signals different repetition rates in response to track occu- Automatic block signal systems with wayside pancy. Each pulse rate is a code to indicate allowa- signals and trip stops, while offering effective train ble train speed. This pulsed d.c. energy is passed protection, have certain operational disadvantages. through the rails, picked up inductively by a receiver (antenna) on the train, and decoded to Sometimes the signals are obscured by fog, rain, or retrieve speed command information, This infor- snow. In such cases, operating rules require that the mation is used to actuate the appropriate cab signal operator consider the signal as displaying its most restrictive aspect and operate the train accordingly. display. Because the train is continuously receiving pulses of energy, a change in the pulse rate of the If the signal is actually displaying a more per- coded track circuits indicating a change of condi- missive indication, time is lost unnecessarily. A sections ahead of the train is instantaneously received ond disadvantage is that wayside signals convey by carborne equipment and displayed by cab signals commands only at the entrance to a block. The train regardless of where the train happens to be within a operator must reduce speed to the maximum permitted by the signal and maintain that speed until block. reaching the next signal. If conditions change im- Figure 6 illustrates how cab signals control a mediately after the train enters the block and it train in a three-block, three-aspect signaling becomes safe to proceed at a greater speed, the train system. In this example, the code rates transmitted operator has no way of knowing this since the sig- through the rails (expressed as pulses per minute) nal is behind him. Again, time is lost. With wayside correspond to the following signal aspects: block signals there is also the possibility that the 180 Green (Proceed) operator will fail to observe the signal correctly, read the wrong signal in multiple-track territory, or 75 Yellow (Proceed at medium speed forget the indication of the last signal passed. If prepared to stop) there are trip stops, these kinds of human failure do 21 not result in an unsafe condition, but the efficiency O Red (stop) of train operation can be adversely affected. One way to overcome these disadvantages is to 21Note that O code-the absence of a code—is the most provide signal displays within the cab of the train, restrictive, Thus, any failure of the track circuit or the carborne This is called cab signaling, A display unit, mounted receiver is a fail-safe condition since it is interpreted by the cab in the cab within the train operator’s forward field signal equipment as a command to stop.

25 HOW IT WORKS: Receiver coils, mounted on the train near the rails, receive pulse-coded track signals, which are decoded and used to pick up relays that energize the cab signal lamp indicating track conditions ahead.

FIGURE 5.-Cab Signals

FIGURE 6.—Three-Block, Three-Aspect Cab Signal System

26 The situation depicted here is the same as in the il- This arrangement allows the train operator to lustration of wayside signals (figure 4). Train B is control speed so long as it does not exceed the com- approaching Train A, which is completely stopped, manded speed shown on the cab signal unit. If the Note that the moment Train A starts to move and commanded speed is exceeded or if the block speed clears the block, Train B receives a green signal im- changes to a lower value because of another train mediately—not at the entrance to the next block, as ahead, the operator receives an audible warning. it would with wayside signals. Note also that a O The operator has a fixed time (typically 2 to 3 sec- code appears in the part of the block immediately onds) to initiate the required braking manually. If behind Train B as it moves along the track and that this is done, the brakes can be released when the Train B can approach closer to Train A before being commanded lower speed is reached. If not, the required to stop, brakes are applied automatically and irrevocably by the ATC system, and the train is brought to a full Speed Control stop before the operator can resume control. This is analogous to the overspeed control provided by With the addition of speed sensing and brake wayside signals with trip stops, except that braking control mechanisms, cab signals can also be used to can be initiated anywhere within a block not just at provide automatic overspeed protection. Figure 7 is the entrance. Another difference is that trip stops a schematic diagram of such a system. It is the same act to stop the train after an overspeed condition has as the schematic shown in figure 5, except for the occurred over a measured course, usually several addition of speed and code rate comparison equip- hundred feet in length. Cab signals do the same, but ment and the direct connections to the propulsion instantaneously, thus eliminating the delay in- and braking systems. herent in the preliminary measured course and per-

Train Wheels & Axle /

FIGURE 7.—Cab Signal System With Automatic Overspeed Protection

27 —

mit trains to follow one another more closely for a transmits a command signal that generates, on given block length. board the train, a velocity-distance profile which the train is to follow to a stop. Additional triggers, Automatic Train Operation nearer the station platform, correct the generated velocity-distance profile for the effects of wheel Basically cab signaling provides carborne slip and slide. The ASR system monitors the automatic train protection in the form of collision velocity-distance profile and controls the braking prevention. With the addition of on-board equip- effort to bring the train to a stop at a predetermined merit for sensing and comparing command (allowa- point. Another method of programed stopping ble) and actual speed, cab signaling makes it possi- makes use of long wayside antenna to provide a ble to expand the train protection function to permit series of position signals to a carborne control speed regulation. This, in turn, forms the basis for system as the train passes along its length. The car- extending automation into the area of train opera- borne control system determines train position and tion. combines this with speed and deceleration informa- Several forms of automatic train operation tion (sensed on board the train), to produce an ap- (ATO) are possible, but all have two basic propriate propulsion or braking command for the features-automatic speed regulation and station traction control system. stopping. To this basic ATO system, other automated Automatic speed regulation (ASR), as the name features may be added. Doors can be opened implies, is basically a comparator circuit for match- automatically after the train is brought to a stop in a ing actual speed to command speed. Speed comands station, This requires a circuit to actuate door open- received from coded track circuits are picked up by ing mechanisms and appropriate safety interlocks to a carborne receiver, decoded, and compared to ac- assure that the train is in fact stopped and at a sta- tual train speed sensed by a tachometer in the drive tion. Door closure may also be automated by adding mechanism. Up to this point, an automatic speed a timing circuit to measure how long the doors have regulation system is like cab signaling. The been open and to initiate a door closure signal difference arises in how this comparison is used. automatically after a predetermined dwell time has With cab signals, the comparison is used to actuate elapsed. Train departure can also be initiated a penalty brake application to stop the train when automatically by introducing another control circuit actual speed exceeds command speed. With ASR, to apply propulsion power after receipt of a signal the comparison is used to control the motors and confirming that doors are closed and locked. brakes in an effort to minimize the difference be- For each of these levels of ATO, the train opera- tween actual and command speed. An advisory dis- tor may be provided with an advisory display to play of speed commands and train speed may be show what commands are being received and what provided for the operator. In effect, ASR removes response is being made by automatic mechanisms. the human operator from the control loop for run- The operator may also be provided with manual ning the train and provides for an essentially instan- override controls to inhibit automatic functions or taneous and invariant response by propulsion and to vary automatic system operation. For example, braking systems, without the delay of human reac- the operator may intervene manually to adjust the tion time and without the variability and possibility stopping point, to prevent some or all doors from for misinterpretation inherent in manual train opening, to vary station dwell time, or to initiate or operation. prevent departure. Figure 8 shows a functional The other basic element of ATO is station stop- diagram of a typical ATO system and a picture of ping, which involves bringing the train to stop the train operator’s console. automatically at a predetermined location in each station. This is accomplished by special wayside control units working in cooperation with position Interlocking receivers, logic circuits, and automatic speed An interlocking is an arrangement of signals and regulation equipment on the train. One method signal appliances so interconnected that functions uses wayside “triggers” spaced some distance from must succeed each other in a predetermined se- the station as reference points for programed stop- quence, thus permitting safe train movements along ping. The first trigger, farthest from the station, a selected route without collision or derailment. An

28 FIGURE 8.—Automatic Train Operation System

29 FIGURE 9.—Typical Interlocking Location

interlocking thus consists of more than just follows automatically, including inhibition of switches to allow trains to move along crossing, further switch movement until the train has tra- merging or branching routes; it is also made up of versed the limits of the interlocking. signals and control devices that automatically pre- A more advanced, but still not completely auto- vent conflicting or improper movements. Interlock- ing may be manually controlled or equipped with mated, type of interlocking is a system that permits automatic devices that sort trains through branches a towerman or central supervisor to select the and junctions according to desired destinations, entrance and exit points for a train to pass through an interlocking, with the switches and signals for Several forms of automatic interlocking are in the appropriate route then being set up use. One of the oldest and simplest is an arrange- automatically by an arrangement of electrical ment of hand-operated switches, each of which relays, Figure 10 shows such a control panel for a controls an individual signal or track turnout. The system called entrance-exit route interlocking, The switches are mechanically or electrically intercon- tower operator moves the control knobs to desig- nected such that once a particular route is selected, nate a desired route. Internal logic circuits the switch points locked in place, and the signals automatically select the best available nonconflict- cleared, no other route for a potentially conflicting ing route, aline and lock switches, and activate the move can be established until the train bound for appropriate wayside signals to allowing train move- the cleared route has safely passed, This arrangement while holding other signals at stop to prevent ment represents a semiautomated form of move- conflicting moves. This level of automation may be ment control, Manual operation is required to select characterized as automatic execution in response to a route and move the control levers, but all else manual inputs,

30 FIGURE 10.—Entrance-Exit Interlocking Control Panel

Fully automatic interlocking are also in use. In Train Supervision Equipment addition to track circuits, switch operation, and sig- Train supervision embraces a wide variety of nal control elements, the automatic interlocking functions. The special-purpose equipment that has must have some device for identifying a specific been developed to perform these functions is train in order to create the necessary input to the 22 equally varied. In a general survey of train control logic circuits. One method to identify trains is by technology it is not possible to describe all types of means of wayside optical device that scans a panel automatic and semiautomatic devices that are in on the lead car which gives destination, route, and use. The following, therefore, is a brief catalog of other needed information. Another method makes some of the more important systems. use of a carborne transponder that is interrogated by a wayside device. With either technique, however, Train dispatching is concerned with the timing of train identity becomes the substitute for manual in- train departures from terminals in accordance with puts that allows trains to be sent along predeter- the schedule of operations. In conventional transit mined routes without human involvement. systems this function is accomplished by preprogrammed dispatching machines that automatically ring a bell or flash a light as a signal 22 A rudimentary form of automatic interlocking is one that to the train operator that it is time to leave a ter- uses a simple in-out logic circuit to switch trains from one track minal or intermediate waypoint. In some systems, to another. This device is commonly used at terminals and oper- ates to switch each entering train from the inbound to the out- the dispatch function may be assigned to a central bound track and thus does not require train identity information. train control computer that transmits electric start-

31 ing signals to the train in accordance with a master taking action to smooth out irregularities of traffic schedule stored in the computer memory. flow. In most transit systems this function is carried out by central control personnel aided by automatic Route assignment and control is a train super- display devices. One such device is a pen recorder visory function that is allied to the train protection that marks a moving paper graph to record the function of route interlocking. Route control is a passage of trains past check points, Each spike on strategic function, consisting of selecting routes for the graph indicates the presence of a train, as trains and transmitting the orders to wayside points, detected by the track circuits, at some time and where the orders are implemented tactically by in- place along the route. A train supervisor, by check- terlocking equipment. In conventional transit ing this graph against the schedule, can monitor the systems, route assignment and control is performed progress of all trains operating on line and detect locally, either manually or automatically, With delays or queuing up of trains, (Figure 18, page 36 remotely controlled route interlocking, however, it shows such a device, ) becomes operationally practical to place the Another form of performance monitoring aid is strategic and tactical management of routing in a the model board (figure 11), which is a schematic computer. The programing to accomplish this is representation of the track plan of the transit relatively simple and straightforward, and a com- system with indicator lights to denote track circuit puter is ideally suited to handle what is an essen- occupancy and, hence, the position of each train on tially repetitious task with a limited number of the line, This is the functional equivalent of the alternative courses of action. The safety aspects of pengraph recorder, but in a more pictorial form of route interlocking are assured not by central com- display. Another type of model board used in newer puter control, but locally by conventional interlock- transit systems has, in addition to the master track ing equipment at the wayside, plan, small cathode ray tube displays that permit individual supervisors to obtain more detailed or ex- Performance monitoring involves comparing the panded views of selected track sections or to call up overall movement of traffic with the schedule and special-purpose presentations of data,

FIGURE Il.—Model Board and Train Control Console

32 Pengraphs, model boards, and the like are not also include provisions for manual inputs and dis- fully automatic supervisory devices. The human plays at central control or on the train, but the nor- operator is still needed to interpret the display and mal mode of operation is automatic. to formulate orders to individual trains. In the most advanced systems routine performance monitoring A WALK is assigned to computers, which keep a continuous THROUGH A TRANSIT SYSTEM watch on traffic movement and automatically calculate and transmit performance commands to To place ATC in perspective, it maybe helpful to trains. Man, in this circumstance, acts in a com- make a brief tour of the facilities of a transit system, pletely different supervisory capacity. He does not pointing out the type and location of the equipment monitor and regulate traffic. Instead, he supervises that carries out train control functions. machines which, in turn, monitor and regulate traffic. Station There are two general types of action that can be The passenger’s first point of contact with a tran- taken to smooth out irregularities in traffic flow. sit system is the station. The most prominent Both are accomplished in response to commands features-vending and fare collection facilities from central control. One is to hold a train in a sta- (possibly automated), escalators and elevators, tion for a time longer or shorter than the scheduled heating and air conditioning, and platform dwell time or, in extreme cases, to direct a train to amenities—have nothing to do with train control. bypass a station in order to close up a gap. The other There may also be public address systems and video method is to alter the speed of the train between or audio surveillance equipment for fare collection stations. This latter method is called performance and platforms. These are not, strictly speaking, part level modification and takes the form of a propor- of the train control facility even though they maybe tional reduction of train speed below the speed nor- connected to the central control facility and mally allowed in each block. In systems supervised monitored by central supervisory personnel, About by a central computer and with automatic train the only direct manifestations of ATC are the auto- operation, performance level modification is ac- mated train departure and destination signs or complished without human intervention. The re- loudspeakers found in some transit systems. These quired reduction is calculated by the central com- public announcement devices are connected to the puter and automatically transmitted to stations or ATC system and use information inputs derived other critical locations, where the signals are picked from track circuits and train identification equip- up by carborne ATO equipment that modifies the ment, There may be an ATC equipment room in the response to the normal speed commands transmit- station, but it is out of sight and locked. Its presence ted by the coded track circuits. These systems may is usually unknown to passengers.

FIGURE 12.—General View of Rail Rapid Transit Station

33 —-——

These are the impedance bonds that isolate the track into blocks. At the ends of the blocks, there are small boxes, containing relays, with electrical connections from the track circuits to the signaling apparatus,

FIGURE 14,—Track Circuit Wiring

Other signal equipment is contained in small cases placed at intervals along the right-of-way. There are also telephones or other communication equipment and antennas or transmitters used for precision station stopping, train identification, or performance level modification, In certain locations, ATC apparatus and other trackside equipment may be housed in small sheds to protect the equipment from the weather and to facilitate maintenance by wayside workers.

FIGURE 13.—Trip Stop

Wayside An observant passenger might notice two wayside features that can be seen from the station platform. Looking down the tracks in the direction of train movement, there are wayside signal lights that change aspect from time to time. Often, just beyond the downstream end of the platform and FIGURE 15.—Wayside Equipment Case (contains multiplex

alongside the rail, there is a trip stop which can be equipment for transmitting information between trains and seen to raise behind a train that has just left the sta- station control rooms) tion and later lower as the train recedes. Moving out along the tracks, other wayside ele- At junctions and crossovers there is switch ap- ments can be found. The track circuits themselves paratus, the most visible parts of which are the are not plainly visible since they are largely in switch points, frogs, levers, and motive equipment, wayside housings. However, at intervals there are This is the wayside equipment, known as a switch small flat equipment cases situated between the machine, that performs the function of interlocking rails and connected to them by electrical wiring. for train protection,

34 FIGURE 16.—Track Apparatus at an Interlocking

By far the largest part of the equipment, train but along the wayside and in central control facilities, and structures along the right-of-way— facilities. trackage, tunnels, bridges, the third rail, and power distribution equipment-are not related to train Central Control control. Nevertheless, the wayside is where the bulk of the ATC equipment in a transit system is lo- Supervisory control of the system may be exer- cated. The proportion varies as a function of the cised in a central control room equipped with model level of automation, but generally about 80 percent boards, communication, equipment, system or more of all train control equipment is not on the monitoring apparatus, and individual supervisor’s

FIGURE 17.-Central Train Control Facility

35 FIGURE 18.—Two Views of a Central Control Facility with Electromechanical Equipment

left-a clock-driven paper tape device for dispatching trains

above-pengraph device for monitoring train movement

Not all transit systems have a single centralized consoles. If the system has ATS, the computers and control facility. Some disperse control and supervi- other data processing equipment are also located in sion to outlying towers, situated at major interlock- the central control building, which often houses ad- ing along the routes. Figure 19 is a photograph of ministrative and training facilities as well. such a local control tower.

FIGURE 19.—Tower for Local Control of Interlocking

36 Vehicles Most of the ATC equipment on transit vehicles is carried in equipment cases under the body or in the train operator’s cab. About the only features distinguishable from outside the train are a receiver coil mounted on the lead car to pick up coded track circuit signals (figure 20) and—for systems with optical scanners-small identification panels mounted on the side of each car.

FIGURE 21.—Train Operator’s Console for System with ATO

Yards and Shops A large part of the important activity of a transit system does not occur in revenue service on the main lines, but in the yards and shops, These facilities, though seldom seen by the riding public, contribute greatly to the quality and level of service that the transit system offers. The yards are usually located near terminals and consist of a vast complex of tracks for storing vehicles and making up trains to be operated on the The operator’s cab contains the displays and con- lines. Even in systems with the most advanced trols necessary to operate the train or to monitor the functions of ATC equipment. The amount and levels of automation, train operation in yards is under manual control. Train sorting and classifica- sophistication of this equipment varies greatly— tion is also an essentially manual operation, ranging from very simple and utilitarian apparatus although some systems have a limited amount of in manually operated systems to highly complex consoles in the newest and most automated automatic switching in the yards, principally to and from revenue tracks. systems, The console typically includes propulsion and brake controls, a speedometer and command Car shops and maintenance facilities are usually speed indicator, lighted placards indicating the located within the yard complex. The shops contain operating state of automatic elements, warning facilities for light and heavy maintenance, compo- lights, pushbuttons or control knobs to make data nent repair, car washing, and checkout of vehicles inputs or to select various operating modes, a train before they are dispatched back into service, phone or radio for communicating with central supervisors, a passenger address microphone, and a The maintenance facility may also include a test deadman control to prevent the train from operat- track and special test equipment to qualify vehicles ing in case the operator is inattentive or incapaci- and components for acceptance or to carry out trials tated. of equipment modifications.

37 ——

FIGURE 22.—Aerial View of Rail Rapid Transit Yard and Maintenance Facility

38 39 ——

LEVELS OF AUTOMATION lowest levels of automation—primarily ATP with some ATS. The newer systems have ATP and ATO It was suggested earlier that train control and more extensive ATS, None are completely automation can be viewed as a continuum. At one automated. extreme, all functions are performed by human operators; at the other, all are performed by Historically, the conversion from manual to machines. The transit systems now in operation or automatic train control in rail rapid transit has been under development in this country lie at various incremental and has followed a more or less com- points between these extremes, with their relative mon course for all systems. These major technologi- positions corresponding roughly to the age of the cal stops along the road to automation are outlined system. The older systems generally have the briefly below and summarized in table 1.

TABLE I.—Levels of Automation

LEVEL CHARACTERISTICS EXAMPLE

Essentially Manual Train protection by rules and procedures CTA Train operation manual (with or without the aid of (Ravenswood and advisory wayside signals) Evanston Lines) Train supervision by towermen and/or central dispatched Wayside Signal Protection Wayside block signals with trip stops for train NYCTA separation and overspeed protection . Train operation manual Supervision manual with some automation of dispatching and route interlocking Carborne Train Protection Cab signals and equipment-enforced train protection CTA Train operation manual Supervision as above Automatic Train Operation Automatic Train Protection as above PATCO Train operation either completely automatic or with manual door operation and train starting Train supervision as above Automatic Train Supervision ATP and ATO as above BART Train supervision automatic (or mostly so) under central computer control Unmanned Operation ATP, ATO, ATS as above AIRTRANS No on-board operator System manned only by small number of central control personnel

Full Automation ATP, ATO, ATS as above, with automatic, not None manual backups for each Skeleton force at central control Yard operation automated

40 Essentially Manual systems and all of the Philadelphia (SEPTA) system also employ this form of automatic train protection. At this level, train protection, operation, and supervision are carried out by train operators and towermen or central supervisors with little or no aid Carborne Train Protection from automatic equipment. Trains are protected and operated either by rules and procedures alone Cab signaling, using coded track circuits and or with the aid of advisory wayside signals. There automatic carborne stopping and speed limit en- are no automatic stop-enforcing mechanisms either forcement, represents the same level of ATP as on the wayside or on board the train. Train dis- wayside signals with trip stops. To this extent, this patching is carried out by personnel at terminals or level of automation is equivalent to the preceding. at control towers along the routes, using either a Generally, however, cab signaling is considered a written schedule or timing devices that act as higher level of automation since it also provides prompters to signal train departure. Route assign- some automatic aids to train operation—principally ment and interlocking control are accomplished by automatic and continuous display of speed informa- manually activated equipment that may have some tion to assist the operator in running the train and automatic safety features but are entirely controlled stopping at stations. Other aspects of train operation by human operators. Communications are by means are still essentially manual. Cab signaling does not of visual signals (lights, hand signals, posted civil necessarily lead to any increase in the automation speeds, etc. ) or by telephone from stations and of supervisory function nor is it accompanied by towers to central control. any change in the communications systems. Many of the older transit systems in this country This level and form of automation is generalIy began operation at the manual level, but they have regarded as the minimum for a new transit system, since advanced to more automated forms of train and most of the older transit systems either have control. One of the last vestiges of a purely manual converted or plan to convert to cab-signaled ATP. system is on the Ravenswood and Evanston lines of the Chicago Transit Authority, which as late as Automatic Train Operation 1975 operated without any automatic block signal protection, The major advantage of cab signaling over wayside signaling is that bringing the speed command on board the train also permits evolution to automatic train operation. All of the information Wayside Train Protection needed to operate the train automatically is either inherent in the cab signal system or readily availa- Wayside signals with trip stops form the basis for ble through modular additions. At this level, the automatic train protection, by assuring separation human is removed from the speed control, station of following trains and preventing conflicting stopping, door control, or starting loops--or any moves at interlocking. Incorporation of timing combination of them. The human no longer func- devices with the trip stops also provides equipment- tions as an operator but as an overseer of carborne enforced overspeed prevention. While train protec- control systems. tion thus becomes automatic, train operation is still completely manual. Train supervision also remains Along with ATO, there is often (but not an essentially manual activity, although track cir- necessarily) an increase in the level of automation cuits and signals used primarily for train protection of train supervisory functions. ATS functions that do permit some automation of route interlocking are sometimes considered operationally desirable to and dispatching—usually in the form of semi- implement at the time ATO is installed include automatic devices (i. e., manually activated but automatic dispatching, route assignment, and per- automatically operating). formance level modification.

All transit systems in the United States have at The two newest transit systems in this country— least this level of automation. The most notable ex- Bay Area Rapid Transit and the Port Authority ample of an entire system with enforced wayside Transit Corporation—both have ATO. The new signaling is the New York City Transit Authority. systems under development in Washington, Atlan- Portions of the Chicago, Boston, and Cleveland ta, and Baltimore will also have it.

41 Automatic Train Supervision reduced in number as more supervisory tasks are allocated to machines. Train supervision functions (except for dispatching and route control) are among the last to be auto- No rail rapid transit system in the United States, mated. To be effective and operationally practical, or anywhere in the world, is now operating at this ATS usually can be introduced only when there is a level of automation. The technology to do this, high level of automation in the areas of ATP and however, is available; and it has been applied in ATO.23 Automatic train supervision also requires a various people-mover systems, such as the Morgan- rather complex and sophisticated communication town Personnel Rapid Transit (PRT) and several network, not only for voice messages but also for airport transportation systems. A notable example the interchange of large quantities of data among of an unmanned airport transit system is automatic system elements on a real-time basis. AIRTRANS at the Dallas-Fort Worth Airport, The distinguishing feature of ATS, however, is the where small unmanned transit vehicles circulate on use of a central computer (or computers) to process fixed guideways over a complex of interconnecting and handle data, make decisions, and formulate in- routes. The entire system is operated and super- structions. vised from a central location by a few persons aided by a train control computer, The Bay Area Rapid Transit system was the first rail rapid transit system to make extensive use of Full Automation ATS. The new Washington, Atlanta, and Baltimore systems will also have highly automated train Complete removal of man from control of transit supervision based on computer control. While there system operation-even removing him from the are some differences among them in the type and central control point—is probably not technically amount of control vested in ATS computers, these feasible or desirable, For safety and continuity of four stand apart from all other transit systems in operation, it will always be necessary to have some- this country in the extent to which automation one to monitor the system and intervene to restore technology is applied to train supervision. operations or assist passengers in an emergency. The number of such supervisors would be only a Unmanned Operation handful, however, and it is doubtful that they could ever conduct normal operations manually as a back- At all the levels of automation described pre- up to automatic systems. viously, there is at least one operator on board each train and some supervisory personnel in central Such a “fully automatic” transit would require control. While these people are not part of the nor- an extremely sophisticated and costly ATC system, mal control loop, they do exercise important func- which would include ATP, ATO, and ATS for nor- tions as overseers of automatic equipment and mal modes of operation and—most important— back-ups in case of failure or emergency; A more automatic back-ups of these mechanisms for con- advanced form of automation is one where the tingencies and emergency states, The communica- trains are unmanned, with all ATP and ATO function network would also have to be highly sophisti- tions performed by automatic devices. The few re- cated, providing not only voluminous real-time in- maining human operators in the system are at terchange among automatic components but also central control, but even these personnel may be extensive two-way voice links between passengers and the supervisory cadre. Another requirement of such a system would be automatic operation, switching, and assembly of trains in yards. The 23 Even with ATP and ATO, ATS is not truly necessary until technology for automatic yard operation is available the demands imposed by the complexity of the route structure today in rudimentary form in automated freight and the required level of service outstrip the capacity for effective real-time supervision by manual methods. ATS may also classification yards, but it would need to be refined become necessary when the load in peak periods approaches 100 extensively before application to a rail rapid transit percent of system capacity. system,